${\mathrm{Yb}}_{18}{\mathrm{Pt}}_{51.1}{\mathrm{Si}}_{15.1}$ $(\approx \mathrm{Yb}{\mathrm{Pt}}_3\mathrm{Si})$ is a representative in the series of $R{\mathrm{Pt}}_{\approx 3}\mathrm{Si}$ compounds. The crystal structure of ${\mathrm{Yb}}_{18}{\mathrm{Pt}}_{51.1}{\mathrm{Si}}_{15.1}$ has been determined from room temperature x-ray single crystal charge coupled device data: with $a=1.86246\left(3\right)\mathrm{nm}$; $c=0.40513\left(1\right)\mathrm{nm}$ and space group $P4∕mbm$. ${\mathrm{Yb}}_{18}{\mathrm{Pt}}_{51.1}{\mathrm{Si}}_{15.1}$ is isotypic with the structures of ${(\mathrm{Y},\mathrm{Dy})}_{18}{\mathrm{Pt}}_{50+x}{\mathrm{Si}}_{16-x}$ revealing a curvilinear arrangement of distorted $\mathrm{Ce}{\mathrm{Pt}}_3\mathrm{Si}$-type fragments around Yb-centered cube-octahedra of ${\mathrm{Pt}}_{12}\left[\mathrm{Yb}\right]$. Similar to the structures of ${(\mathrm{Y},\mathrm{Dy})}_{18}{\mathrm{Pt}}_{50+x}{\mathrm{Si}}_{16-x}$ misfit regions with strong distortions are encountered as a typical feature of heavy rare-earth compounds $R{\mathrm{Pt}}_{\approx 3}\mathrm{Si}$. On cooling, ${\mathrm{Yb}}_{18}{\mathrm{Pt}}_{51.1}{\mathrm{Si}}_{15.1}$ orders antiferromagnetically below $1.8\mathrm{K}$, followed by a second magnetic phase transition at $500\mathrm{mK}$. Physical properties arise from the mutual influence of Ruderman-Kittel-Kasuya-Yosida interaction, the Kondo effect and crystal electric field (CEF) splitting, with the first excited CEF doublet more than $100\mathrm{K}$ above the ground state. The magnetic field dependence of the low-temperature heat capacity gives rise to a large magnetocaloric effect around $2\mathrm{K}$.

• Received 8 August 2005

DOI:https://doi.org/10.1103/PhysRevB.73.104405